1. Field of the Invention
The present invention relates to a back-light device for a liquid crystal display (LCD) and more particularly to a side-light type back-light device for LCDs provided with a cathode tube on the side of a light guiding plate.
2. Description of the Related Art
In recent years, instead of a cathode-ray tube (CRT) and the like, a liquid crystal display (LCD) is widely employed as a display device because it requires less space.
Configurations and functions of the back-light device for LCDs are hereinafter described by referring to FIGS. 1 and 9. FIG. 1 is an exploded view explaining configurations of a side-light type back-light device for LCDs according to a first embodiment of the present invention. FIG. 9 is a sectional view explaining configurations of a conventional back-light device.
As shown in FIG. 1, the back-light device for LCDs of the present invention is comprised of a cathode tube 5 used as a light source for the back light, a cathode tube reflection plate 4 to reflect light emitted by the cathode tube 5, a reflection sheet 2 to reflect light toward the direction of an LCD panel, a light guiding plate 1 to guide light to a liquid crystal display surface and a diffusion sheet 6 to diffuse light given by the light guiding plate 1. Light from the cathode tube 5 reflected off the cathode tube reflection plate 4 is repeatedly reflected between the light guiding plate 1 and the reflection sheet 2, and then enters, while being scattered, the LCD panel as uniform light.
As a display screen of the LCD of this kind is increasing in size, it becomes necessary to make smaller a region other than a display region, i.e., a rim of the display screen as much as possible. If, however, the rim of the display screen is made smaller, since light reflected off the reflection sheet 2 or the cathode tube reflection plate 4 enters more a region being near to the cathode tube 5 on the LCD screen compared with other regions, a bright line composed of line-like light is produced, causing the impairment of the display quality of the LCD screen.
A method for preventing such bright lines is disclosed in Japanese Laid-open Patent Application No. Hei9-244019 in which a gray or black reflection preventing film is provided in a portion, positioned nearer to the cathode tube, of the reflection sheet. This method is explained below by referring to FIG. 9. Light (shown by dotted lines) being radiated on the end of a portion, which is positioned nearer to the cathode tube 5, of the reflection sheet 2, out of light emitted from the cathode tube 5, enters a region, positioned nearer to the cathode tube 5, of the LCD panel and tends to form bright lines if no reflection preventing film 8 is mounted. However, in the configurations as shown in FIG. 9, since the reflection preventing film 8 is mounted at the end of the reflection sheet 2, light entering this reflection preventing film 8 does not return to the light guiding plate 1, thus eliminating the occurrence of bright lines.
In LCDs, it is important to adjust chromaticity according to various applications. The methods for adjusting chromaticity includes changing a color of the cathode 5 itself, adding new members or parts such as a chromaticity correcting filter or the like to the reflection sheet 2 or to the cathode tube refection plate 4 or additionally performing a coloring process.
However, such conventional technologies present problems. That is, it is required in LCDs that the occurrence of bright lines can be prevented, the chromaticity can be easily adjusted and the moderate brightness can be maintained. Though, in the conventional methods as described above, the occurrence of bright lines can be prevented by providing the reflection preventing film 8 to the portion, positioned nearer to the cathode tube 5, of the reflection sheet 2, there is a problem that the brightness is lowered because light entering the reflection preventing film 8 cannot be effectively used.
Moreover, in the method where the color of the cathode tube 5 is changed to adjust the chromaticity, it is necessary to prepare various kinds of the cathode tubes being different in the chromaticity and, therefore, a great number of man-hours and much time are needed. Also, in the method where the new parts such as the chromaticity correcting filter or the like are added to the whole of the reflection sheet 2 or cathode tube reflection plate 4, or where coloring processes are performed on the whole of the reflection sheet 2 or cathode tube reflection plate 4, a reflection efficiency rate is lowered and, though the chromaticity is adjusted, the brightness is lowered.
In view of the above, it is an object of the present invention to provide a back-light device for a liquid crystal display (LCD) in which chromaticity can be corrected at low cost and desired back-light color can be obtained without impairing the brightness of an LCD panel and in which the occurrence of bright lines can be effectively prevented.
According to a first aspect of the present invention, there is provided a back-light device for a liquid crystal display comprising:
a light guiding plate;
a diffusion sheet mounted in proximity to a surface on one side of the light guiding plate, which light being transmitted through the surface on one side of the light guiding plate out of light incident on the light guiding plate;
a reflection sheet mounted in proximity to a surface on other side of the light guiding plate, which is used to reflect light reflected off the surface of one side of the light guiding plate;
a chromaticity correcting member for correcting chromaticity provided at a part of a region where light incident on the light guiding plate passes or is reflected; and
whereby the light reflected off a reflection sheet is reflected repeatedly between the surface on one side of the light guiding plate and the reflection sheet, causing light incident on the light guiding plate to be uniformly radiated over a liquid crystal display panel.
According to a second aspect of the present invention, there is provided a back-light device for a liquid crystal display comprising:
a light guiding plate;
a cathode tube used to emit light into the light guiding plate;
a diffusion sheet mounted in proximity to a surface on one side of the light guiding plate, which light being transmitted through the surface on one side of the light guiding plate out of light incident on the light guiding plate;
a reflection sheet mounted in proximity to a surface on other side of the light guiding plate, which is used to reflect light reflected off the surface on one side of the light guiding plate;
a chromaticity correcting member for correcting chromaticity mounted at the end, positioned nearer to the cathode tube, of the reflection sheet; and
whereby the light reflected off the reflection sheet is reflected repeatedly between the surface on one side of the light guiding plate and the reflection sheet, causing light incident on the light guiding plate to be uniformly radiated over a liquid crystal display panel.
According to a third aspect of the present invention, there is provided a back-light device for a liquid crystal display comprising:
a light guiding plate;
a cathode tube used to emit light into the light guiding plate;
a diffusion sheet mounted in proximity to a surface on one side of the light guiding plate, which light being transmitted through the surface on one side of the light guiding plate out of light incident on the light guiding plate;
a reflection sheet mounted in proximity to a surface on other side of the light guiding plate, which is used to reflect light reflected off the surface on one side of the light guiding plate; and
a chromaticity correcting member for correcting chromaticity mounted at the end, positioned nearer to the cathode tube, of a surface on which the diffusion sheet is attached;
whereby the light reflected off the reflection sheet is reflected repeatedly between the surface on one side of the light guiding plate and the reflection sheet, causing light incident on the light guiding plate to be uniformly radiated over a liquid crystal display panel.
In the foregoing, a preferable mode is one wherein the chromaticity correcting member for correcting chromaticity is provided with a reflection film which reflects only light having a predetermined wavelength.
Also, a preferable mode is one wherein the chromaticity correcting member for correcting chromaticity contains a plurality of roughly semi-spherical scattering bodies which scatters and reflects only light having a predetermined wavelength.